1. Field of the Invention
This invention relates to ink jet print heads and more specifically to a method for modifying ink jet print heads to prevent degradation of ink contact angles after continued exposure to molten phase change inks.
2. Background Art
Ink jet printers having one or more ink jet print heads with one or more ink jetting nozzles in each printhead for projecting drops of ink to generate graphic images and text have become increasingly popular. To form color images, ink jet printers with multiple ink jetting nozzles are used, with each nozzle being supplied with ink of a different color. These colored inks are then applied, either alone or in a combination, to the printing medium to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan, magenta, and yellow inks. Black ink may also be added to the above ink combination when the combination of the cyan, magenta and yellow does not produce a true enough black, or when text is being printed.
Various systems and methods are known for producing printed images with aqueous based inks. A serious problem in printing images with ink jets that use aqueous based inks is wetting of the ink discharge surface. Wetting of the discharge surface is caused by a low ink contact angle, and typically ink contact angles of greater than 90.degree. are sought. The ink contact angle is the angle formed by the tangent to the ink drop at the ink discharge surface and the ink discharge surface. The ink contact angle is created by a difference in surface energies between the ink composition and the material defining the discharge surface. The larger the ink contact angle, the less wetting of the discharge surface that occurs.
The presence of ink deposits due to surface wetting on the ink discharge surface surrounding the drop discharge nozzle causes several problems. The most severe problem is that the wetted surface eventually degrades the ink contact angle between the ejecting ink droplet and the discharge surface such that no ink is discharged at all. This becomes a more prevalent problem as the rate of ink ejection is increased. Another problem caused by wetting of the discharge surface is that the ink deposits cause non-uniform ink ejection or off-axis shooting. Non-uniform ink ejection causes poor quality of the printed image. Still another problem caused by wetting of the discharge surface is that a color ink jet print head may have nozzles of different colors adjacent to each other. As the discharge surface wets, the colors mix and the ink droplets become contaminated, which also leads to poor quality of the final printed image.
Various methods or approaches have been developed which treat the discharge surface of an ink jet system with non-wetting materials thereby preventing deposits of ink from spreading out across the discharge surface from the drop discharge nozzles of the ink jet system. This is accomplished by using a coating material which has a very low surface energy with respect to the surface energy of the ink being used. The difference in surface energy causes the ink contact angle between the coated discharge surface and the ink to be greater than when no coating is used. With a larger ink contact angle, the ink drop that forms is more likely to be completely ejected, thus less ink is left on the discharge surface to begin the wetting process. Some examples of these various methods and approaches for treating the discharge surface of an ink jet head are described below.
In U.S. Pat. No. 4,533,569, Aug. 6, 1985, of Bangs for PROCESS PREVENTING AIR BUBBLE LOCK IN INK JET NOZZLES, the interior surface area of a glass nozzle is cleaned with hydrofluoric acid and then coated with a blocking agent such as ethylene glycol, glycerine and the like. Anti-wetting compounds, such as long chain anionic non-wetting agents, are applied to the fluid nozzles after ionic pretreatment to improve ink drop quality.
U.S. Pat. No. 4,623,906, Nov. 18, 1986 of Chandrashekhar et al. for STABLE SURFACE COATING FOR INK JET NOZZLES, describes a three-layer coating for glass or silicon ink jet nozzles comprising silicon nitride and/or aluminum nitride.
In U.S. Pat. No. 4,343,013, Aug. 3, 1982, of Bader et al. for NOZZLE PLATE FOR INK JET PRINT HEAD, the nozzle plate of an ink jet printer, which is made of glass, is coated with a material which is non-wetting relative to the aqueous characteristics of the ink composition. Compositions such as tetrafluoroethylene or certain silicone based materials are useful for this purpose since they have these aforementioned non-wetting characteristics.
A liquid repellant film layer of a fluorosilicon non-wetting compound is provided on the surface area surrounding the jet nozzle in U.S. Pat. No. 4,368,476, Jan. 11, 1983, Uehara et al. for INK JET RECORDING HEAD.
In U.S. Pat. No. 4,643,948, Feb. 17, 1987, of Diaz et al. for COATINGS FOR INK JET NOZZLES, an ink jet nozzle plate is coated with a non-wetting film of a partially fluorinated alkyl silane and a perfluorinated alkane, respectively.
A nozzle plate of the electrostatic ink jet printer is polished to a mirror finish and then is completely coated with a thin layer of Teflon.RTM. resin in U.S. Pat. No. 4,728,393. However, in this case, the Teflon.RTM. coating is employed for electrostatic control, not for ink drop formation. Ink drop formation is facilitated by the air-assist and mesa mechanisms. For this reason the ink jet would work without the Teflon.RTM. coating.
In U.S. Pat. No. 3,946,398, Mar. 23, 1976, of Kyser et al. for METHOD AND APPARATUS FOR RECORDING WITH WRITING FLUIDS AND DROP PROJECTION MEANS THEREFOR, an ink contact angle of greater than 90.degree. between the ink and the drop ejection surface is desired to prevent ink wetting. This angle is obtained by using aqueous inks and by coating the drop ejection surface with a Teflon.RTM. coating. However, no method for applying the Teflon.RTM. coating is described.
An article related to application of a fluorocarbon polymeric film, "Highly Non-Wettable Surface Plasma Polymer Vapor Deposition of Tetrafluoroethlyene" by B. D. Washo, in the IBM TDB, Vol. 26, No. 4, Pg. 2074, describes the benefits of having a toughened surface to maximize contact angles and thus reduce wetting when contact angles greater than 90.degree. exist. Another article relates to the application of a Teflon.RTM. layer to a surface surrounding a nozzle. This article, "Preventing Clogging of Small Orifices in Objects Being Coated" by W. W. Hildenbrand and S. A. Manning, in the IBM TDB, Vol. 15, No. 9, Pg. 2899 (February 1973), describes how to prevent the clogging of a nozzle by ejecting nitrogen through the nozzle so that the nitrogen flows out of the nozzle while the Teflon.RTM. layer is being sprayed on to the surface.
However, all of the above mentioned references relate to the problems encountered with the use of aqueous-based inks. In a different ink jet printing technology, non-aqueous, phase change inks have been employed in place of aqueous-based inks in ink jet systems. A phase change ink is solid at room temperature but becomes liquid at the elevated operating temperature of the ink jet so that it may be jetted as liquid drops in a predetermined pattern. The jetted ink then solidifies and forms the image. The problems caused by wetting of the drop ejection surface described above in relation to aqueous-based inks occur with phase change inks as well. However, there are a few major differences between phase change inks and aqueous-based inks that cause problems with regard to discharge surface wetting that are not solved by the aforementioned teachings.
First, after continued exposure to the molten ink at the elevated operating temperatures of a phase change ink jet head, the anti-wetting properties of the non-wetting surface start to degrade and even the 60.degree. contact angles become difficult to maintain. As the ink contact angle decreases, wetting of the surface becomes more prevalent. Eventually, the ink contact angle decreases to the point where the wetting of the discharge surface causes the ink jet nozzle to fail to eject an ink drop. Furthermore, any non-wetting material within the ink jet nozzle causes off-axis shooting, and may even prevent the jetting of ink from the nozzle. The off-axis shooting typically occurs because the difference in surface energy between the ink composition and the non-wetting material creates a large ink contact angle within the nozzle.
Second, because the ink contact angle with phase change ink is smaller than with aqueous-based ink, more wetting of the discharge surface occurs. Therefore, the type of process for cleaning a phase change ink jet head is more destructive to a coating material that is applied to the discharge surface than the cleaning processes typically used with aqueous-based ink jet printers. It has been noted that after repeated cleaning, the coating material starts to wear off of the discharge surface. Furthermore, any grooves, valleys, or gross differences in thicknesses on the discharge surface allow wetted ink to gather. If these differences are severe enough, ink is left on the discharge after the cleaning process.
Therefore, a method is needed for applying an anti-wetting coating to an ink jet head such that the ink contact angles do not degrade after continued exposure to molten phase change inks at the high operating temperature of such a print head. Furthermore, there is a need for a method of applying an anti-wetting coating to a phase change ink jet head such that no coating material remains within the nozzle of the ink jet head. Still further, a method is needed for applying an anti-wetting coating to a phase change ink jet head such that the coating does not chip off or wear off the surface during operation of the ink jet printer. Still further, there is a need for a method of applying an anti-wetting coating to a phase change ink jet head such that the surface is smooth. These problems are solved by the method of the present invention.